The cells of the hematopoietic system arise from multipotent progenitors, the hematopoietic stem cells (HSCs), which progress through a series of developmental programs to ultimately form the terminally differentiated cells of the myeloid or lymphoid lineage. It is believed that in the initial stages of hematopoiesis, HSCs commit to two distinguishable oligopotent but developmentally restricted progenitor cell types, the common lymphoid progenitors (CLPs) and the common myeloid progenitor (CMPs). T lymphocytes, B lymphocytes, natural killer (NK) cells, and lymphoid dendritic cells develop from corresponding progenitor cells derived from the CLPs whereas erythroid cells, megakaryocytes, granulocytes, macrophages, and myeloid dendritic cells develop from their corresponding progenitor cells derived from CMPs. Cell populations at each stage of differentiation are distinguishable from other cell populations in the hematopoietic pathway based on programmed expression of a unique set of cell markers.
Although HSCs are capable of self renewal—cell division that results in at least one of the daughter cells having the same characteristics as the parent cell—the progenitor cells committed to the lymphoid or myeloid lineages lose their potential to self-renew. That is, mitotic cell division of the committed progenitors leads to differentiated progeny rather than generation of a cell with the same proliferative and differentiation capacity as the parent cell. This loss of self-renewal potential is seen in the ability of committed progenitors cells to maintain hematopoiesis only for a limited time period (i.e., short term reconstitution) following transplantation of the progenitor cells into an immunocompromised animal, as compared to an HSC, which can completely regenerate and maintain hematopoiesis during the life of the host animal (i.e., long term reconstitution).
It has been observed, however, that in certain disease states of the hematopoietic system, dysregulation of cellular regulatory pathways may lead to progenitor cells that acquire the ability to self-renew. For instance, acute myeloid leukemia (AML, also called acute myelogenous leukemia) is a myeloproliferative disorder marked, in part, by infiltration of bone marrow by abnormal hematopoietic cells. Indeed, the stem cell nature of cancer was first shown in AML (Lapidot et al., 1994 Nature 17:645-8). AML is categorized into different subtypes based on morphological features and cytochemical staining properties, and although the self-renewal characteristic in most types of AML is attributable to leukemic cells having cell marker phenotypes consistent with HSCs (Bonnet, D. and Dick, J. E., Nat. Med. 3(7):730-737 (1997)), the chromosomal abnormality associated with the AML M3 subtype is observed in cell populations with a cell marker phenotype characteristic of more differentiated cells of the myeloid lineage (CD34−, CD38+) whereas the HSC population in M3 does not carry the translocation (Turhan, A. G. et al., Blood 79:2154-2161 (1995)).
Gain of self-renewing characteristic in the committed progenitor cell population is also suggested in chronic myeloid leukemia (CML, also called chronic myelogenous leukemia, or chronic granulocytic leukemia), a disease commonly associated with the Philadelphia chromosome, which is a balanced translocation between chromosomes 9 and 22, t(9;22). The translocation produces a fusion between the bcr and c-abl genes and results in expression of a chimeric protein BCR-ABL with increased tyrosine kinase activity. Although the HSC population in CML typically contains the chromosomal abnormality, the BCR-ABL fusion protein is mainly expressed in the committed cells of myelomonocytic lineage rather than the HSCs, indicating that committed cells in the myeloid lineage may be the source of the leukemic cells rather than the HSCs. Additional evidence for the committed myeloid cells as being the source of the leukemic clones in CML comes from studies of controlled expression of BCR-ABL in transgenic animals. Use of promoters active specifically in myeloid progenitor cells to force expression of BCR-ABL in committed cells but not in HSCs produces disease characteristic of CML in these transgenic animal models (Jaiswal. S. et al., Proc. Natl. Acad. Sci. USA 100:10002-10007 (2003)).
Although myeloproliferative disorders, such as AML and CML are typically associated with cytogenetic abnormalities, the cytogenetic defect may not be solely responsible for the proliferative trait. In some instances, the chromosomal abnormality is observed in normal cells, which suggests that accumulation of additional mutations in either the HSCs or committed myeloid cells is required for full manifestation of the disease state. Even in CML, the disorder displays a multiphasic course, beginning from a chronic phase, which after 3-5 years and up to 10 years, leads to an accelerated or blastic phase similar to AML. The time period required to transition from the chronic phase (less than 5% blasts or promyelocytes) toa the blastic phase (>30% blasts in the peripheral blood or bone marrow) may reflect the time needed to accumulate the mutations responsible for conversion of the chronic phase to the more aggressive blastic phase. For the most part, however, the leukemic cells appear to retain the cell marker phenotypes detectable in normal progenitor cells.
Treatments for proliferative disorders normally rely on the sensitivity of proliferating cells to cytotoxic or cytostatic chemotherapeutic agents. For instance, busulfan, a bifunctional alkylating agent, and hydroxyurea, an inhibitor of ribonucleoside diphosphate, affect DNA synthesis and stability, resulting in toxicity to dividing cells. Other therapeutic agents of similar activity include cytosine arabinoside (cytarabine) and daunorubicin. However, the effects of these agents are non-discriminatory and as a result they have serious side effects due to toxicity to normal dividing cells.
Another treatment used in patients with haematological malignancies is bone marrow transplant (BMT), where the recipient's hematopoietic cells are eliminated with radiation and/or chemotherapy (e.g., cyclophosphamide), and the hematopoietic system reconstituted by transplant of healthy hematopoietic stem cells. Typically, the transplant uses HLA matched allogeneic bone marrow cells from a family member (HLA-identical) or a serologically matched altruistic donor (MUD). Approximately, <50% of recipients find a donor, with exactly matching histocompatibility. Transplant with less well matched donors marketed increases the transplant related morbidity and mortality. This therapeutic approach has limited application because of its dependence on the availability of suitable donors and because the treatments show better outcome for patients in the chronic or early phase of the disease as compared to acute or late stages.
Antibody therapy for cancer involves the use of antibodies, or antibody fragments, against an antigen to target antigen-expressing tumor cells. Because antibody therapy targets cells expressing a particular antigen, there is a possibility of cross-reactivity with normal cells and can lead to detrimental results. Substantial efforts have been directed to finding tumor-specific antigens. Tumor-specific antigens are found almost exclusively on tumors or are expressed at a greater level in tumor cells than the corresponding normal cells. Thus, tumor-specific antigens provide targets for antibody targeting of cancer, or other disease-related, cells expressing the antigen, as well as providing markers for diagnosis, for example, by identifying increased levels of expression. In immunotherapy approaches, antibodies specific to such tumor-specific antigens can be conjugated to cytotoxic compounds or can be used alone in immunotherapy.
Immunotherapy as a treatment option against hematpoietic cancers, such as AML, is limited by the lack of tumor-associated antigens that are tumor-specific and that are shared among diverse patients. It is desirable to find other therapeutic agents that take advantage of the developmental origins of the leukemic cells by exploiting the common characteristics between leukemic cells and normal cell populations in the myeloid lineage. This approach would provide treatments that can supplement traditional therapies for myeloid leukemias, or that can be used as an alternative treatment to directly target the stem cell fractions of leukemic cells. This approach also provides additional diagnostic and prognostic strategies, as well as strategies for monitoring the efficacy of a therapeutic regimen.
Generally, therapeutic treatment is more effective when tailored to a specific type of hematopoietic cancer. Predicting and determining efficacy of a treatment regime over time is also valuable in terms of clinical management. It is thus desirable to find tumor-specific markers that can be used in more efficient and accurate diagnosis and prognosis of myeloiod leukemic disorders, such as AML.
Cytokine receptors belong to families of receptor proteins, which are divided into two subsets on the basis of the presence or absence of particular sequence motifs. The two subsets are the hematopoietin-receptor family (also referred to as the class I cytokine receptor family) and the class II cytokine receptor superfamily (many of which are receptors for interferons or interferon-like cytokines). In the hematopoietin-receptor family, the α chain often defines ligand specificity of the receptor and the β or γ chain initiates intracellular signaling.